We label by ai the probability that, starting in state i, the chain is eventually absorbed in the closed set {0,3}. Since {0,3} is closed,
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a₀ = 1 (if you start in 0 you’re already in {0,3}),
-
a₃ = 1,
and the other closed state 1 lies outside our target class, so -
a₁ = 0.
States 2 and 4 are transient, and satisfy the usual linear equations
a₂ = P(2→0)·a₀ + P(2→1)·a₁
a₄ = Σₖ P(4→k)·aₖ.
From the matrix row for state 2, P(2→0)=1/3, P(2→1)=2/3, and no chance to go directly to 3 or 4. Hence
a₂ = (1/3)·1 + (2/3)·0 = 1/3.
From the row for state 4, the transitions are
-
to 0 with probability 1/8,
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to 1 with probability 1/8,
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to 2 with probability 1/2,
-
to 3 with probability 1/8,
-
to 4 itself with probability 1/8.
Thus
a₄ = (1/8)·a₀ + (1/8)·a₁ + (1/2)·a₂ + (1/8)·a₃ + (1/8)·a₄
Substitute a₀=1, a₁=0, a₂=1/3, a₃=1:
a₄ = 1/8 + 0 + (1/2)·(1/3) + 1/8 + (1/8)·a₄
a₄ = 1/8 + 1/6 + 1/8 + (1/8)a₄
Combine the constants: 1/8+1/8 =1/4, and 1/4+1/6 = (3/12+2/12)=5/12, so
a₄ = 5/12 + (1/8)·a₄
Bring the a₄–term to the left:
a₄·(1 – 1/8) = 5/12
(7/8)·a₄ = 5/12
a₄ = (5/12)·(8/7) = 40/84 = 10/21.
Hence starting from state 4 the probability of eventual absorption in {0,3} is 10/21.